1. Field of the Invention
This invention relates generally to maintenance of a control rod drive of a boiling water reactor. Specifically, the invention relates to a tool for dismantling or assembling a portion of a control rod drive during a maintenance operation.
2. Description of the Related Art
Control rod drives are used to position control rods in boiling water reactors to control the fission rate and fission density, and to provide adequate excess negative reactivity to shut down the reactor from any normal operating or accident condition at the most reactive time in core life. FIG. 1 is a sectional view of a boiling water nuclear reactor pressure vessel 10 with parts cut away to expose the interior thereof and illustrates the environment in which a control rod drive operates. The reactor pressure vessel 10 has a generally cylindrical shape and is closed at one end by a bottom head 12 and at its other end by a removable top head 14. A sidewall 16 extends from the bottom head 12 to the top head 14. A cylindrically shaped core shroud 20 surrounds a reactor core 22. The shroud 20 is supported at one end by a shroud support 24 and includes a removable shroud head 26 at the other end. An annulus 28 is formed between the shroud 20 and the sidewall of the vessel 16.
Heat is generated within the core 22, which includes fuel bundles 36 of fissionable material. Water circulated up through the core 22 is at least partially converted to steam. Steam separators 38 separate steam from water, which is recirculated. Residual water is removed from the steam by steam dryers 40. The steam exits the reactor pressure vessel 10 through a steam outlet 42 near the vessel top head 14 and is commonly used to drive a turbine generator for the production of electricity.
The fuel bundles 36 are aligned by a lower core plate 50 located at the base of the core 22. A top guide 52 aligns the fuel bundles 36 as they are lowered into the core 22. Core plate 50 and top guide 52 are supported by the core shroud 20.
The amount of heat generated in the core 22 is regulated by inserting and withdrawing control rods 44 of neutron absorbing material, such as hafnium. To the extent that the control rods 44 are inserted between the fuel bundles 36, the control rods absorb neutrons that would otherwise be available to promote the chain reaction which generates heat in the core 22. The control rod guide tubes 46 below the lower core plate 50, align the vertical motion of the control rods 44 during insertion and withdrawal. Hydraulic control rod drives 48, which extend through the bottom head 12, effect the insertion and withdrawal of the control rods 44. A better view of a control rod drive is shown in FIGS. 2 and 3. Referring to the sectional view shown in FIG. 2, each control rod drive is mounted vertically in a control rod drive housing 18 which is welded to a stud tube 30, which in turn is welded to the bottom head of the reactor vessel 12. A control rod drive flange 32 is bolted and sealed to the flange 34 of the control rod drive housing 18, which contains ports for attaching the control rod drive hydraulic system lines 54, 56. Demineralized water supplied by the control rod drive hydraulic system serves as the hydraulic fluid for effecting control rod drive operation.
As shown in FIG. 2, the control rod drive 48 is a double acting mechanically latched hydraulic cylinder. A spud 62 at the top of the index tube 64 (the moving element) engages and locks into a socket at the bottom of the control rod. Once coupled, the control rod drive and control rod form an integral unit which must be manually uncoupled by specific procedures before the control rod drive or control rod may be removed from the reactor.
As shown in FIG. 3, which provides a cross sectional view of the upper portion of a control rod drive, the spud 62, which connects the control rod to the control rod drive, is threaded onto the upper end 66 of the index tube 64 and held in place by locking band 68. Six spring fingers 70 permit the spud 62 to enter a mating socket on the control rod. A lock plug (not shown) then enters the spud from the socket and prevents uncoupling. The control rod can be uncoupled by lifting the lock plug by raising an uncoupling rod consisting of rod 72 and tube 74.
An outer filter 76 and an inner filter 78 are installed near the upper end of the control rod drive. Both are provided to filter reactor water flowing in the control rod drive to remove foreign particles or abrasive matter that could result in internal damage and excessive wear. The outer filter 76 consists of a ring with a flange 80 on its outer periphery, a perforated cylinder for supporting a woven wire filter cloth and a guide, all welded together. The outer filter 76 is installed on the control rod drive by three lock wire screws 82 which secure the lower end of the outer filter 76 to guide cap 84. The outer filter 76 removes foreign particles from reactor water entering the annulus between the control rod drive outer tube and a thermal sleeve (not shown) in the reactor vessel control rod drive housing.
Control rod drives are frequently removed from the reactor for servicing in order to maintain their reliability. During maintenance of a control rod drive, the outer filter is removed. This filter has been exposed to fields of radiation during reactor operation. Historically, long handled tongs that interact with the filter only and are pounded off with a weighted sleeve on the shaft have been used to remove the outer filter after the three filter mounting screws have been removed. Using the current tool, it is difficult to maintain a grip on the filter. Furthermore, it is difficult to hold the tool in place and operate the weighted sleeve at the same time. Furthermore, the current tool requires the operator to be in an awkward position which can lead to injury. Further, using the current tool, there is no way to create a parallel pulling force on the filter which causes binding, thus increasing removal time. The foregoing difficulties lead to a prolonged removal time which results in more radiation dose for an operator using the existing tool. While improved tools have been suggested for this purpose, such as that described in U.S. Pat. No. 5,347,554, they appear bulky, difficult to position, and do not provide a mechanical advantage to assist removing the filter.
Accordingly, a new tool is desired that is easier to handle and provides a mechanical advantage in creating an axial removal force that withdraws the outer filter from the control rod drive.